1. Field of the Invention
The present invention relates to a rotational angle detector used for detecting a rotational angle of a steering wheel of a car, for example.
2. Background Art
Recently, functions of cars have been improved, and thus more cars have detected rotational angles of the steering wheels using rotational angle detectors to perform various controls. A conventional rotational angle detector is described with reference to FIG. 4. FIG. 4 is an exploded perspective view of an essential part of the conventional rotational angle detector.
Rotary body 21 has spur gear 21A in its outer periphery, and has engaging part 21B engaging with a steering shaft (not shown) that penetrates its center. Detecting unit 22 has spur gear 22A in its outer periphery, and detecting unit 24 has spur gear 24A in its outer periphery. Spur gear 22A meshes with spur gear 21A, and spur gear 24A meshes with spur gear 22A. Magnet 23A and magnet 25A are mounted to the center of detecting unit 22 and the center of detecting unit 24 by insert molding or the like, respectively. Of rotary body 21, detecting unit 22, and detecting unit 24, the diameter of gear and the number of teeth of rotary body 21 are the largest, those of detecting unit 22 are intermediate, and those of detecting unit 24 are the smallest.
Wiring board 28 disposed over and in substantially parallel with detecting units 22 and 24 has a plurality of wiring patterns (not shown) on its upper and lower surfaces. Magnetic detecting elements 23B and 25B are mounted to wiring board 28 so that they face magnets 23A and 25A, respectively. Wiring board 28 has controller 29 formed of an electronic component such as a microcomputer. Controller 29 is connected to magnetic detecting elements 23B and 25B.
Thus, the rotational angle detector has the following elements:                a first detecting element including magnet 23A and magnetic detecting element 23B;        a second detecting element including magnet 25A and magnetic detecting element 25B; and        controller 29.        
Controller 29 is connected to an electronic circuit (not shown) of a car body through a connector (not shown) or the like, and the steering shaft penetrates engaging part 21B of rotary body 21. In this way, the rotational angle detector is mounted to a car.
When a driver rotates the steering wheel during driving, rotary body 21 rotates, detecting unit 22 rotates in cooperation with rotary body 21, and detecting unit 24 rotates in cooperation with detecting unit 22. Magnets 23A and 25A also rotate in cooperation with these rotations. Magnetic detecting elements 23B and 25B detect changing magnetic forces of magnets 23A and 25A as detection signals composed of sine and cosine waveforms of voltage. These detection signals have sawtooth data waveforms and are fed into controller 29. Since the number of teeth and rotation speed of detecting unit 22 differ from those of detecting unit 24, respective data waveforms of the detection signals of detecting unit 22 and detecting unit 24 have different cycles and shifted phases with each other.
Controller 29 performs a predetermined calculation based on two different detection signals supplied from magnetic detecting elements 23B and 25B and each number of teeth, and detects the rotational angle of rotary body 21. That is, controller 29 detects the rotational angle of the steering wheel. The detected signals are supplied to the electronic circuit of the car body, and are used for various controls of the car. Such a rotational angle detector is disclosed in Japanese Patent Unexamined Publication No. 2005-3625, for example.
In this conventional rotational angle detector, however, when detecting unit 22 or detecting unit 24 is chipped or broken, controller 29 detects a false rotational angle based on the detection signals from magnetic detecting elements 23B and 25B. In order to address the problem and to attain a long service life by preventing chipping or breakage through strengthening detecting units 22 and 24, the size of detecting units 22 and 24 must be enlarged or a material of high strength must be used.